A primary use of oscilloscopes is to measure the amplitude of an electrical signal. Where the signal is oscillatory in nature, the amplitude measured from the signal's highest point to its lowest point (the peak-to-peak amplitude) may be of particular interest. This information can be obtained by the operator by simply reading the waveform displayed on the oscilloscope screen. For many oscilloscope tasks, however, this peak-to-peak amplitude must be provided internally to the machine, such as for scaling the voltage to display the entire signal amplitude over a preselected portion of the oscilloscope screen.
Traditionally, measuring the amplitude has required the operator to display manually at least one cycle of the signal as a waveform on the screen and then measure the amplitude by visually comparing the waveform against the graticule impressed on the screen. This measurement, however, is only the first step. The operator must then determine the units (volts, milliamperes, etc.) per graticule division and multiply by the number of divisions and fractions thereof to obtain the signal's amplitude.
This manual method although simple to use suffers from several drawbacks. For one, the information obtained is not provided internally to the instrument for performing other tasks. For a second, the accuracy of the information is poor because the operator can easily misread the graticule and the graticule resolution is limited. For a third reason, multiplying mentally the volts per division by the determined divisions and fractions thereof is subject to error. Finally, the method takes considerable time for the operator to position himself properly, read the graticule, determine the volts per division, perform the multiplication, and repeat the steps again to check his result.
In an attempt to improve on the manual method, a method for automatically measuring the amplitude has been devised for oscilloscopes currently in use, such as on the real time oscilloscope model number 2465 presently manufactured and sold by the assignee of this patent. This method utilizes a single trigger circuit of the oscilloscope to measure the peak-to-peak amplitude. The trigger level of the circuit is adjusted until the circuit no longer triggers on the signal, thereby indicating the maximum peak value of the signal. The trigger level is then adjusted until again the circuit no longer triggers, thereby indicating the minimum peak value of the signal. The second value is then subtracted from the first to obtain the peak-to-peak amplitude.
Although an improvement over the prior manual method, this automatic method is still relatively slow. The delay is significant for tasks such as automatically adjusting the volts per division to display the entire signal amplitude in a preselected portion of the screen. Since the amplitude measurement forms the foundation from any other oscilloscope tasks as well, this delay has a considerable effect on the scope's overall performance.